This invention relates to polyamide polymers which become brittle after long exposure to moisture, and to blends of such polyamides and certain water soluble polymers which rapidly lose mechanical properties when exposed to moisture. It also relates to use of such polymers as degradable monofilament, fibers, degradable plastic articles, and degradable films. It further relates to the use of such polymers in biological implants.
Japanese Kokai No. 56 022324 describes the production of alternate block copolymers of low molecular weight aliphatic polyesters and low molecular weight aliphatic polyamides, by ester-amide interchange, which are useful for mulch films biodegradable via enzyme digestion (Rhizopus delemar lipase). In this polymer, the polyester to polyamide ratio is high in polyester, for example, 4 to 1 mole per cent polyester to nylon 6.
Japanese Kokai No. 54 119594 describes the production of a biodegradable low molecular weight aliphatic polyester amide alternate block copolymer from polycaprolactone and nylon 6 in the presence of zinc acetate. The polyester to polyamide ratio is near 1 to 1. Again, the product polymer is subject to enzymatic (lipase) degradation.
Japanese Kokai No. 54 120727 describes polyester polyamide block copolymers, high in the polyester component, which are useful for production of biodegradable films or fibers. There is no mention of marine uses of the copolymers.
U.S. Pat. No. 3,592,873 describes the preparation of polyamide esters as interpolymers to provide thermal stability in polyoxymethylenes. The polyamide esters are prepared by reacting lactams or alkyl substituted lactams with at least a four membered ring with lactones or alkyl substituted lactones, again with at least a four membered ring.
J. Appl. Polymer Sci., 24(7), 1701-11 (1979) describes the synthesis of copolyamide-esters via amide ester interchange of polycaprolactone with nylon 6, 66, 69, 11, 12, or 612. It also describes biodegradation of the polymers by Rhizopus delemar lipase digestion or alcoholic alkali hydrolysis. The effect of nylon/polycaprolactone ratio on the biodegradability was examined and biodegradability was found to decrease as the polycaprolactone content decreased. Furthermore, biodegradability was also found to decrease as the length of the polyamide blocks shortened. Copolyamide-esters with less than about 20% ester content were found to be non-biodegradable.
Eur. Polym. J., 529-557 (1984) describes the preparation of copolyesteramides by anionic ring opening copolymerization of .epsilon.-caprolactam with .epsilon.-caprolactone. Alternating copolymers or random multiblock copolymers with amide to ester ratios of 90/10 to 10/90 were prepared. In addition, the reference describes cleavage by alkaline hydrolysis as well as tensile properties of films and fibers fabricated from the copolymers.
Chemtech, 21, 26-30 (January, 1991) describes the biodegradability of a variety of commercial plastics and compares their utility for medical applications. Medical uses are related to mechanical and degradative properties of many of the commercial materials.
The combination of properties which polyamides exhibit make them ideal for use in fibers. The term "polyamide" in this application includes any aliphatic or cycloaliphatic polyamide. Such materials are known commercially as nylons. Nylon fibers have many properties which make them ideal for use in netting, including strength, light weight, and resistance to degradation, However, some of these desirable properties also result in a significant environmental problem. The lifetime for nylon netting in the ocean has been estimated as over ten years, and may be closer to thirty years. Netting which has either been abandoned intentionally or by accident continues to capture marine fish and mammals. These "ghost" nets account for enormous kills of fish, seals, whales, and dolphins. Seal deaths alone are estimated to be about 40,000 annually world wide. A similar situation is found in abandoned lobster traps where any captured lobsters are unable to free themselves from the trap's netting. Analogous situations exist in fresh water lakes, many being contaminated with fishing line either lost or abandoned.
Commercial nylons are also used in biological implants. However, because of their biological inertness, uses are limited to those in which the implant is permanent or in which it can be mechanically removed.
Polymers of lactic acid are well-known for their degradability under microbial attack. A copolyester of lactic and glycolic acids is used as a biodegradable suture in repairing soft tissue wounds. These polymers have also been used to fabricate degradable bone plate which is used to reinforce a broken bone during its healing period. There is no record of lactic acid/nylon copolymers being used in a similar manner.
Oxalate substitution for adipate units in nylon 66, or similar units in other bi-directional polyamides for the purpose of rendering the material more degradable is not reported. Oxalate esters are highly reactive hydrolytically. Thus, incorporation of oxalate units into nylon provides sites for attack and chain cleavage, whether biologically or by simple hydrolysis.
It is an object of this invention to create a degradable nylon which will degrade at a controlled rate in fresh water, ocean water, or other humid environments. It is a further object of this invention to create a degradable nylon useful for biological implants. Another objective is to create a degradable nylon film. A still further object is to provide a process for the efficient production of the degradable nylon.